1. Field of the Invention The present invention relates to a multi-carrier wireless communication system and, in particular, to a method and apparatus for transmitting uplink control information on data channels in a wireless communication system supporting multicarrier transmission.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) is applied to most broadcast and wireless communication systems. OFDM is advantageous for wireless communication because of the robustness to the multipath fading channel, guaranteeing orthogonality between multiple access users, and spectrum efficiency. Due to these advantages, OFDM is considered as one of the most attractive transmission techniques for high speed and broadband communication system and even superior to the Direct Sequence-Code Divisional Multiple Access (DS-CDMA) technique. However, the high peak to average power ratio (PAPR) of the OFDM increases the power consumption, and this may decrease the coverage. For this reason, the 3GPP Long Term Evolution (LTE) uses the OFDM for the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink to increase the coverage and reduce the power consumption of the mobile terminal. Since both the OFDM and SC-FDMA are multiplexing the users or channels in frequency domain, there is similarity in allocating frequency resources as the scheduling resources.
FIG. 1 is a diagram illustrating a structured of a subframe carrying Uplink Control Information (UCI) in the conventional LTE system, and FIG. 2 is a diagram illustrating a structure of a subframe carrying the UCI and data in the conventional LTE system.
The UCI includes the Acknowledgement/Negative-Acknowledgement (ACK/NACK) information related to data packet received in the downlink, Channel Quality Indicator (CQI) reports, and Rank Indicator (RI) information, and is transmitted on a Physical Uplink Control Channel (PUCCH). As shown in
FIG. 1, the PUCCH 101 and 102 is transmitted on reserved frequency regions at the edges of the total available bandwidth in the uplink. In this case, when the mobile terminal transmits the packet data, the reserved frequency region cannot be used for PUCCH. This is because the simultaneous transmission of the PUCCH for the UCI and the Physical Uplink Shared Channel (PUSCH) for the data does not fulfill the single carrier characteristic and thus increases the PAPR.
In the current LTE system, the UCI is transmitted on the frequency resource allocated for the data, i.e. PUSCH, in the duration for transmitting the uplink data as shown in FIG. 2. In case of transmitting the UCI on the PUSCH resource, the multiplexing scheme is changed depending on the characteristic of the control information. That is, the CQI information is rate-matched, attached at the tail of data bits, and mapped to physical bits, thereby arranged at a rear position 107 of the subframe; and the ACK/NACK information is arranged at positions 106 where the data bits are punctured at both sides of the reference symbol. The RI information is arranged at both sides of the reference symbol as the ACK/NACK information between the data bits rather than puncturing the data bits. Since some parts of the resource allocated for the data transmission are used regardless of the transmission scheme, the data transmission amount is reduced as much as the resource used for transmitting the UCI.
How the resource amount for transmitting the UCI, when transmitting the UCI along with the data on the PUCCH resource, is described hereinafter.
Here, the resource means a number of symbols or a number of bits allocated for the PUSCH.
In case of transmitting the UCI on the PUCCH, the information bits are channel-coded such that the number of bits to be transmitted on the PUCCH is fixed per type of UCI. By increasing or decreasing the transmission power, the receipt quality can be maintained at a target level. In case of transmitting the UCI with the data on the PUSCH, the transmission power for the UCI should be equal to that for the data. If the data are transmitted in high spectral efficiency or high Modulation or at high Coding Scheme (MCS) level, the received Signal to Noise Ratio (SNR) per symbol increases; and otherwise, if the data are transmitted in low spectral efficiency or low MCS level, the received SNR per symbol decreases. In order to maintain the reception quality of the UCI, a number of transmission symbols for the UCI is needed to be adjusted in consideration of the data. In LTE, the number of transmission symbols required for the UCI is adjusted according to the spectral efficiency of the data transmitted on the PUSCH. This can be expressed by an equation. When the number of symbols available for transmitting the UCI is Q′, Q′ can be obtained using an equation. The number of symbols for transmitting the ACK/NACK or RI information is calculated as equation (1):
                              Q          ′                =                  min          (                                    ⌈                                                O                  ·                                      M                    sc                    PUSCH                                    ·                                      N                    symb                    PUSCH                                    ·                                      β                    offset                    PUSCH                                                                                        ∑                                          r                      =                      0                                                              C                      -                      1                                                        ⁢                                      K                    r                                                              ⌉                        ,                          4              ·                              M                sc                                  PUSCH                  -                  current                                                              )                                    (        1        )            
where O denotes a number of bits of the ACK/NACK information or Rank Indicator (RI) information, MSCPUSCH denotes a number of subcarriers allocated for the transmission of PUSCH, NsymbPUSCH denotes a number of SC-FDMA symbols for transmitting PUSCH, and Kr is a number of data bits before channel coding. All these parameters are obtained from the PDCCH received at the initial transmission. βoffsetPUSCH denotes an offset value for taking into consideration of the difference between target SNRs of the data and UCI.
Given a number of symbols, a number of bits to be channel-coded of each UCI can be obtained by equation (2) in consideration of the modulation scheme.QACK=Qm·Q′  (2)where Qm is a value indicating the modulation scheme (set to ‘2’ for QPSK and ‘4’ for 16 QAM).
The equation for the CQI information is similar to that for the ACK/NACK information and RI basically. However, since a Cyclic Redundancy Check (CRC) can be added for the CQI large in size and the RI is always assigned the resource, the equation is modified for the remained resource to fulfill the minimum resource amount for the CQI information as equation (3):
                              Q          ′                =                  min          (                                    ⌈                                                                    (                                          O                      +                      L                                        )                                    ·                                      M                    sc                    PUSCH                                    ·                                      N                    symb                    PUSCH                                    ·                                      β                    offset                    PUSCH                                                                                        ∑                                          r                      =                      0                                                              C                      -                      1                                                        ⁢                                      K                    r                                                              ⌉                        ,                                                            M                  sc                                      PUSCH                    -                    current                                                  ·                                  N                  symb                                      PUSCH                    -                    current                                                              -                                                Q                  RI                                                  Q                  m                                                              )                                    (        3        )            where L denotes a number of CRC bits that are not inserted when O is equal to or less than 11 bits but inserted when O is greater than 11 bits, thereby being defined by
  L  =      {                                        0                                              O              ≤              11                                                            8                                otherwise                              .      MscPUSCH−current, NsymbPUSCH−current means numbers of subcarriers and SC-FDMA symbols constituting the subframe. QRI denotes a number of bits for the RI information. Given the number of symbols, the number of CQI bits after channel coding according to the modulation scheme used for the CQI is calculated as equation (4):QCQI=Qm·Q′  (4)
A multi-carrier transmission principle for the LTE-Advanced (LTE-A) is described hereinafter. In the current LTE system, a cell transmits multiple subcarriers on a single carrier, and the mobile terminal also transmits on a single carrier. In the LTE-A system, however, multiple carriers can be aggregated to increase the maximum transmission rate so as to provide spectral efficiency. Nevertheless, the respective carriers maintained in LTE structure to support legacy LTE terminals, and these carriers are called Component Carriers and aggregated to extend the entire bandwidth.
FIG. 3 is a diagram illustrating a principle of carrier aggregation in the LTE-A system. In FIG. 3, four Component Carriers are depicted exemplarily. Each of the four Component Carriers 201, 202, 203, and 204 has the reserved region at the edges for the PUCCH 205 and 206 and the data regions for the PDCCH region at the center.
Accordingly, there is a need for a method for a terminal to transmit the UCI and data simultaneously in the wireless communication system supporting the carrier aggregation. That is, although the LTE-A system uses the PUSCH resource for the transmission of the UCI and data as in the LTE system, there is a need to consider how to assign the resource for the UCI to support the carrier aggregation where the PUSCH is assigned multiple component carriers. There is therefor a need to define how to multiplex the UCI into multiple PUSCH resource, i.e. how many PUSCH symbols to be assigned for the UCI transmission.